Techniques to time vary pilot locations in wireless networks

ABSTRACT

An embodiment of the present invention provides an apparatus, comprising a receiver 5 capable of selecting optimal pilot locations and providing feedback of the pilot locations to a transmitter in communication with the receiver. The optimal pilot locations may be selected by locations that avoid strong interference or platform noise at the receiver, by locations that avoid deep fading, by locations that maximize the spacing between pilot tones at the two ends of a wireless frequency band or by locations that equalize the interspacing between any two adjacent pilots.

BACKGROUND

Wireless communications, including wireless networks, have becomepervasive throughout society. Improvements in wireless communicationsare vital to increase their reliability and speed. Present wirelesscommunication standards, such as but not limited to, current 802.11a/gand TGnSync proposals, may use pilot locations that are constant overtime. Namely, a fixed set of subcarriers may be assigned for pilots forthe whole system all the time. This may cause a problem in the presenceof platform noise and co-channel narrow band interference. Because thereceiver can't gain a synchronization signal from the pilots for acertain period, the receiver's PLL loses synchronization with thetransmitter, and thus the packets may get lost.

Extensive measurements demonstrate that mobile platform noise may be asignificant factor of WLAN performance degradation. Further, theperformance of 802.11a loses more than 20 dB if one of the pilot tonesis corrupted by platform noise or co-channel interference of −83 dBm. Incontrast, the corruption of data tones with the same interference poweronly causes a 10 dB lose. Simulation results for 802.11n demonstratethat loss of pilot tones due to fading significantly degradesperformance by 0.5-6 dB.

Thus, a strong need exists for techniques to time vary pilot locationsin wireless networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the invention is particularly pointed outand distinctly claimed in the concluding portion of the specification.The invention, however, both as to organization and method of operation,together with objects, features, and advantages thereof, may best beunderstood by reference to the following detailed description when readwith the accompanying drawings in which:

FIG. 1 provides a table with SNR loss under narrow band interference ofan embodiment of the present invention;

FIG. 2 illustrates the loss of two pilots in two spatial streams due tofading;

FIG. 3 illustrates the loss of one pilot due to platform noise orco-channel interference;

FIG. 4 is an illustration of one technique to time vary pilot locationswith feedback in a wireless network of one embodiment of the presentinvention;

FIG. 5 is an illustration of one technique to time vary pilot locationsin a wireless network without feed back and with pilot patterns of oneembodiment of the present invention;

FIG. 6 depicts how a transmitter may vary the pilot location across OFDMsymbols of one embodiment of the present invention.

FIG. 7 illustrates spreading pilots across frequency, time and antenna.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the figures have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for clarity. Further, whereconsidered appropriate, reference numerals have been repeated among thefigures to indicate corresponding or analogous elements.

DETAILED DESCRIPTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those skilled in the art that thepresent invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components and circuitshave not been described in detail so as not to obscure the presentinvention.

An algorithm, technique or process is here, and generally, considered tobe a self-consistent sequence of acts or operations leading to a desiredresult. These include physical manipulations of physical quantities.Usually, though not necessarily, these quantities take the form ofelectrical or magnetic signals capable of being stored, transferred,combined, compared, and otherwise manipulated. It has proven convenientat times, principally for reasons of common usage, to refer to thesesignals as bits, values, elements, symbols, characters, terms, numbersor the like. It should be understood, however, that all of these andsimilar terms are to be associated with the appropriate physicalquantities and are merely convenient labels applied to these quantities.

Embodiments of the present invention may include apparatuses forperforming the operations herein. An apparatus may be speciallyconstructed for the desired purposes, or it may comprise a generalpurpose computing device selectively activated or reconfigured by aprogram stored in the device. Such a program may be stored on a storagemedium, such as, but not limited to, any type of disk including floppydisks, optical disks, compact disc read only memories (CD-ROMs),magnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), electrically programmable read-only memories (EPROMs),electrically erasable and programmable read only memories (EEPROMs),magnetic or optical cards, or any other type of media suitable forstoring electronic instructions, and capable of being coupled to asystem bus for a computing device.

The processes and displays presented herein are not inherently relatedto any particular computing device or other apparatus. Various generalpurpose systems may be used with programs in accordance with theteachings herein, or it may prove convenient to construct a morespecialized apparatus to perform the desired method. The desiredstructure for a variety of these systems will appear from thedescription below. In addition, embodiments of the present invention arenot described with reference to any particular programming language. Itwill be appreciated that a variety of programming languages may be usedto implement the teachings of the invention as described herein. Inaddition, it should be understood that operations, capabilities, andfeatures described herein may be implemented with any combination ofhardware (discrete or integrated circuits) and software.

Use of the terms “coupled” and “connected”, along with theirderivatives, may be used. It should be understood that these terms arenot intended as synonyms for each other. Rather, in particularembodiments, “connected” may be used to indicate that two or moreelements are in direct physical or electrical contact with each other.“Coupled” my be used to indicated that two or more elements are ineither direct or indirect (with other intervening elements between them)physical or electrical contact with each other, and/or that the two ormore elements co-operate or interact with each other (e.g. as in a causeand effect relationship).

It should be understood that embodiments of the present invention may beused in a variety of applications. Although the present invention is notlimited in this respect, the devices disclosed herein may be used inmany apparatuses such as in the transmitters and receivers of a radiosystem. Radio systems intended to be included within the scope of thepresent invention include, by way of example only, cellularradiotelephone communication systems, satellite communication systems,two-way radio communication systems, one-way pagers, two-way pagers,personal communication systems (PCS), personal digital assistants(PDA's), wireless local area networks (WLAN), personal area networks(PAN, and the like), wireless wide are networks (WWAN) and Meshnetworks.

Some embodiments of the present invention provide pilot techniques thatare robust to interference and fading. In an embodiment of the presentinvention, the transmitter may vary pilot tone locations. The receivermay feed back the pilot tone locations to the sender according to thereceiver's interference or fading profile. The pilots may be located atthe tones with higher channel gains and less interference. Forsimplicity, the variation of pilot location may be parameterized by astep size of shift in order to maintain the relative spacing betweenpilot tones. In another embodiment of the present invention, the sendermay change the pilot locations across OFDM symbols. Three advantages arethus provided: First, it prevents the pilot from constantly gettinglost. Second, frequency diversity may be obtained. Finally, frequencyresolution of channel tracking may be increased. Thus, a major benefitof the techniques provided herein is that the number of pilot tones maybe reduced and the throughput may be increased.

In current wireless standards, such as but not limited to, 802.11a/g andTGnSync proposal to IEEE 802.11n, pilot locations are constant. Namely,a fixed set of subcarriers are assigned for pilots for the whole systemall the time. This causes a problem in the presence of platform noiseand co-channel narrow band interference. Because the receiver can't gainsynchronization signal from the pilots for a certain period, thereceiver's phase-locked loop (PLL) loses synchronization with thetransmitter, and thus the packets may get lost. Measurements demonstratethat mobile platform noise is significant factor of WLAN performancedegradation. Further, the performance of 802.11g loses 20 dB if one ofthe pilot tones is corrupted by platform noise or co-channelinterference. In contrast, the corruption of data tones with the sameinterference power only causes a 10 dB lose.

Turning now to the figures, FIG. 1 at 100 shows a table illustrating theSNR loss 105 with narrow band interference 110. Interference frequencyof 2439.1875 MHz 120 corresponds to one of the pilot tones in channel 6.On the other hand 2438 MHz 115 interferer only interferes with datatones.

Although not limited in this respect nor limited to 802.11g, the pilottones in 802.11g band may occupy 13*4*0.3125=16.25 MHz in the 76 MHzspectrum of the ISM band. Therefore, there is a 16.25/76=21% chance forany narrow band interferer to corrupt pilot tones. Experiments show thatthere are more than a few narrowband interferers in 802.11 g band,making a corrupted pilot a highly probable reality. Simulation resultsfor 802.11n also demonstrate that loss of pilot tones due to fadingsignificantly degrades performance by 0.3-6 dB.

In FIG. 2 at 200, the effect of fading on pilot tones is illustrated.The four pilots may be located on two tones and two spatial channels 205and 210. Since there is typically some correlation in the receivedsignal of two antennas due to space limitation and platform coupling, itis likely that a pilot tone experiences destructive fading on bothspatial channels. Conventional receivers estimate frequency offset usingat least two pilots on two different tones per OFDM symbol. If one pilottone is faded, the frequency offset can't be estimated using the currentOFDM symbol (even though the estimation may be still possible usingmultiple OFDM symbols including the previous).

Turning now to FIG. 3, the effect of platform noise and co-channelinterference 305 on pilot tones are illustrated generally at 300 as afunction of frequency 315. Platform noise 305 consists of variousharmonics of various clocks in the host computer of the receiver whileco-channel interference 305 includes other narrow band signals (such asBluetooth) located in the same wireless local area network (WLAN) band.These two impairments may corrupt the pilot tone 310 and causesynchronization problems.

Depicted in FIG. 4 at 400, is an embodiment of the present inventionwhich provides two robust pilot schemes, one with feedback and the otherwithout. A first technique that provides a receiver first select optimallocations is shown graphically as a function of frequency 430, withmobile pilots selected as shown at 425 and data tones and DC tones at410, 420 and 415 respectively. The criterion that may be used is asfollows—although the present invention is not limited to thesecriterions.

-   1. Avoid strong interference or platform noise at the receiver.-   2. Avoid deep fading. Criterions 1 and 2 improve the signal to noise    plus interference of the received pilots.-   3. Subjected to criterions 1 and 2, maximize the spacing between    pilot tones at the two ends of the band. This optimizes the    estimation of frequency offset because the estimation of frequency    offset improves as the spacing between pilots increases.-   4. Subjected to criterions 1 and 2, equalize the interspacing    between any two adjacent pilots. This optimizes channel tracking    because even pilot locations are desired for channel interpolation.    However, if this conflicts with criterion 3 when there are more than    two pilots, criterion 3 has a higher priority.

After the pilot locations are selected, the receiver may feed back thepilot locations in a piggy back way such as sending the feedback by anACK packet. In order to reduce the amount of feedback, a set of pilotlocation patterns may be defined and the receiver may only feed back theindex of the selected location pattern. The transmitter may include thefeed back index in the transmitting frame to acknowledge received pilotpattern and indicate the current pilot pattern. An example is shown inFIG. 5, where four pilot patterns 505, 510, 515 and 520 are defined. Thereceiver selects one out of four choices in favor of itschannel/interference status, where each pattern has two pilot locations.The feedback only needs two bits. If the feedback is not available, thetransmitter may employ a default pattern. It is understood that thepresent invention is not limited to four choices and this is providedfor illustrative purposes only.

In another embodiment of the present invention, in the case thatfeedback is not available; the transmitter may vary the pilot locationacross OFDM symbols. Even though the pilots are lost in some symbols,they are recovered in the other symbols. Two examples are illustrated inFIG. 6, at 600 in frequency 605 and 610 vs. OFDM Symbol 615 and 620;although the present invention is not limited to these examples.

In yet another embedment of the present invention spreading the pilotacross frequency and time may be extended to frequency, time, andantenna. For systems employing multiple transmit antennas, the pilot maybe spread out across antennas to avoid destructive fading at thereceiver. As illustrated in FIG. 7, generally as 700, spreading pilotsacross frequency, time, and antenna may be utilized and have beenconsidered in the Institute for Electronic and Electrical Engineers(IEEE 802.16e) standard. However, the purpose is for channel estimationand not phase tracking of PLL (or phase offset estimation). Therequirement in the IEEE 802.16e standard is to uniformly spread out thepilot in frequency, time, and antenna but in an embodiment of thepresent invention no uniformity is required. In the followingdescription as it relates to FIG. 7, pilots sent by antenna 2 aredepicted in the shading shown as 770 and pilots sent by antenna 1 aredepicted in the shading shown as 765,

In an embodiment of the present invention the, pilots of the same OFDMsymbol may separate apart to help phase offset estimation as shown in(d) 760 (pilot sent by antenna 1 is shown as 755, and pilot sent byantenna 2 is illustrated at 750). In figure (a) 705 (pilot sent byantenna 1 is shown as 722, and pilot sent by antenna 2 is illustrated at720), (b) 710 (pilot sent by antenna 1 is shown as 730, and pilot sentby antenna 2 is illustrated at 725) and (d) 760 (pilot sent by antenna 1is shown as 755, and pilot sent by antenna 2 is illustrated at 750), thepilots may occupy the one subcarrier for two (or more) adjacent OFDMsymbols, because space-time coding in the data subcarriers may use thesame subcarrier across adjacent OFDM symbols. This is not required forphase offset estimation as shown in (c) 715 (pilot sent by antenna 1 isshown as 745, and pilot sent by antenna 2 is illustrated at 740). Inaddition, two pilots for the same OFDM symbol may be sent by the same ordifferent antennas for phase offset estimation as shown in (c) 715 and(d) 760.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those skilled in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. An apparatus, comprising: a receiver capable of selecting optimalpilot locations and providing feedback of said pilot locations to atransmitter in communication with said receiver.
 2. The apparatus ofclaim 1, wherein said optimal pilot locations are selected by locationsthat avoid strong interference or platform noise at said receiver. 3.The apparatus of claim 2, wherein said optimal pilot locations areselected by locations that avoid deep fading.
 4. The apparatus of claim3, wherein said optimal pilot locations are selected by locations thatmaximize the spacing between pilot tones at the two ends of a wirelessfrequency band.
 5. The apparatus of claim 3, wherein said optimal pilotlocations are selected by locations that equalize the interspacingbetween any two adjacent pilots.
 6. The apparatus of claim 1, whereinthe providing feedback of said pilot locations to a transmitter incommunication with said receive is accomplished in a piggy back method.7. The apparatus of claim 6, wherein said piggy back method is sendingthe feedback by an ACK packet.
 8. The apparatus of claim 6, wherein saidfeedback is accomplished by defining a set of pilot location patternsand wherein said receiver only feeds back the index of the selectedlocation pattern.
 9. The apparatus of claim 8, wherein if said feedbackis not available, said transmitter may employ a default pattern.
 10. Theapparatus of claim 8, wherein if said feedback is not available, saidtransmitter varies said pilot location across OFDM symbols.
 11. Amethod, comprising: selecting optimal pilot locations by a receiver; andproviding feedback of said pilot locations to a transmitter incommunication with said receiver.
 12. The method of claim 11, furthercomprising selecting pilot locations that avoid strong interference orplatform noise at said receiver.
 13. The method of claim 12, furthercomprising selecting pilot locations that avoid deep fading.
 14. Themethod of claim 13, further comprising selecting pilot locations thatmaximize the spacing between pilot tones at the two ends of a wirelessfrequency band.
 15. The method of claim 13, further comprising selectingpilot locations that equalize the interspacing between any two adjacentpilots.
 16. The method of claim 11, wherein the providing feedback ofsaid pilot locations to a transmitter in communication with said receiveis accomplished in a piggy back method.
 17. The method of claim 16,further comprising sending the feedback by an ACK packet as said piggyback method.
 18. The method of claim 16, wherein said feedback isaccomplished by defining a set of pilot location patterns and whereinsaid receiver only feeds back the index of the selected locationpattern.
 19. The method of claim 18, further comprising employing adefault pattern if said feedback is not available.
 20. The apparatus ofclaim 18, further comprising varying said pilot location across OFDMsymbols if said feedback is not available.
 21. A machine-accessiblemedium that provides instructions, which when accessed, cause a machineto perform operations comprising: selecting optimal pilot locations by areceiver; and providing feedback of said pilot locations to atransmitter in communication with said receiver.
 22. Themachine-accessible medium that provides instructions of claim 21,further comprising said instructions causing said machine to selectpilot locations that avoid strong interference or platform noise at saidreceiver.
 23. The machine-accessible medium that provides instructionsof claim 22, further comprising said instructions causing said machineto select pilot locations that avoid deep fading.
 24. Themachine-accessible medium that provides instructions of claim 23,further comprising said instructions causing said machine to selectpilot locations that maximize the spacing between pilot tones at the twoends of a wireless frequency band.
 25. The machine-accessible mediumthat provides instructions of claim 23, further comprising saidinstructions causing said machine to select pilot locations thatequalize interspacing between any two adjacent pilots.
 26. Themachine-accessible medium that provides instructions of claim 21,wherein the providing feedback of said pilot locations to a transmitterin communication with said receive is accomplished in a piggy backmethod.
 27. The machine-accessible medium that provides instructions ofclaim 26, further comprising said instructions causing said machine tosend the feedback by an ACK packet as said piggy back method.
 28. Themachine-accessible medium that provides instructions of claim 26,wherein said feedback is accomplished by defining a set of pilotlocation patterns and wherein said receiver only feeds back the index ofthe selected location pattern.
 29. The machine-accessible medium thatprovides instructions of claim 28, further comprising said instructionscausing said machine to employ a default pattern if said feedback is notavailable.
 30. The machine-accessible medium that provides instructionsof claim 28, further comprising said instructions causing said machineto vary said pilot location across OFDM symbols if said feedback is notavailable.
 31. A system, comprising: a transmitter capable oftransmitting via a dipole antenna associated with said transmitter; areceiver in communication with said transmitter and capable of selectingoptimal pilot locations and providing feedback of said pilot locationsto said transmitter.
 32. The system of claim 31, wherein said optimalpilot location are selected by locations that avoid strong interferenceor platform noise at said receiver.
 33. The system of claim 32, whereinsaid optimal pilot location are selected by locations that avoid deepfading.
 34. The system of claim 33, wherein said optimal pilot locationsare selected by locations that maximize the spacing between pilot tonesat the two ends of a wireless frequency band.
 35. The system of claim33, wherein said optimal pilot locations are selected by locations thatequalize the interspacing between any two adjacent pilots.
 36. Theapparatus of claim 10, wherein if said feedback is not available, saidtransmitter spreads said pilot across frequency, time, and antenna. 37.The apparatus of claim 36, wherein said spreading of said pilot acrossfrequency, time, and antenna may be done non-uniformly.
 38. Theapparatus of claim 36, wherein two pilots for the same OFDM symbol arecapable of being sent by the same or different antennas for phase offsetestimation.